Method for Eye-Safe Laser Welding

ABSTRACT

A welding tool is configured with a housing enclosing a fiber laser system which is operative to produce a weld seam for connecting two workpieces. The fiber laser system includes a focusing optic configured to focus the output beam of the system so that it propagates through an elongated slit formed in the bottom of the housing. The fiber laser system is capable to move along a predetermined path extending parallel to the longitudinal direction of the slit and limited by the perimeter thereof. The output beam od generated only when the slit sits upon at least one of the workpieces.

PRIORITY CLAIM

This application claims priority from a PCT/EP2009/059831 based on aGerman Patent application 08104923.1 with a filing date Jul. 30, 2008.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates to a laser welding tool for producing a weld seamto join work pieces wherein the laser beam can be moved along the weldseam using a linear drive.

2. Prior Art Discussion

Resistance welding is still the dominant process used in the automotiveindustry mostly using robot-guided welding tongs. Resistance welding isa welding process for electrically conductive materials based on theJoule heat of an electric current flowing through the joint. The Jouleeffect heats the work pieces to be joined until they melt. The weldedjoint is created when the molten material re-solidifies. The work piecesto be joined are usually pressed together using welding tongs during andafter the current flow, which helps generate a homogeneous joint. Laserbeam welding is gaining importance as an alternative process. Laser beamwelding is mostly used for welding work pieces together that have to bejoined with a narrow weld seam and low thermal delay. Like resistancewelding, laser beam welding or laser welding is performed without addinganother material.

The work piece surfaces of the abutting edges or the abutment of thework pieces to be welded is in the immediate vicinity of the focus ofthe collimator that is also called the cross-over point in laser beamwelding. The typical diameter of the cross-over point is between 0.5 and1.0 mm, which produces very high energy concentrations. As it absorbsthe laser output, temperatures on the work piece surface rise extremelyfast above the melting temperature of the metal so that a melt isformed. However, the advantages of laser beam welding are put intoperspective by high acquisition costs, low efficiency and asconsiderable outlay for safety equipment. In addition, the gas orsolid-state laser welding apparatuses currently used in industrialapplications have fairly large dimensions and are not suitable for usein the automotive industry. An example of a laser welding system whereinthe laser beams can be moved along a weld seam using a linear drive isknown, for example, from JP 2004-243393.

A need therefore exists for a fiber laser welding tool that can beintegrated into existing infrastructures for resistance welding and inparticular facilitates the continued use of existing component tools andhandling equipment for welding tools.

A further need exists for a fiber laser welding tool that ensures therequired safety of people at a laser welding output typical forautomotive applications (up to 2 kW) and reduces design-relateddisadvantages of industrial welding tools, in particular highacquisition costs, poor efficiency, and considerable expenses for safetyequipment.

SUMMARY OF THE DISCLOSURE

These needs are satisfied by the disclosed fiber laser that includes adelivery fiber, an optic for coupling the laser radiation from aradiation source into the fiber and a collimator connected to the fiberfor focusing the path of the laser beams. The collimator is arranged onthe linear drive that facilitates movement thereof along a predeterminedline of motion. The linear drive and the collimator are provided in ahousing having an exit slit for the laser beams, wherein the line ofmotion of the collimator runs in parallel to the exit slit. Thecollimator is orientated such that the laser beams exclusively exit fromthe housing through the exit slit and the laser beam can only beactivated when the exit slit of the housing sits upon one of the workpieces.

Use of fiber lasers with compact dimensions and flexibly designablelaser sources allows integration of the laser welding tools into theinfrastructure of conventional resistance welding systems for theautomotive industry. Using the fiber for light guidance, the laserradiation can thus be conducted like over a cable to where the weld seamis to be produced such that laser welding can be performed in confinedspaces as are often encountered in the automotive industry. At the sametime, fiber lasers are characterized by a higher beam quality ascompared to gas and solid-state lasers, improved efficiency of currently30 percent, lower costs of maintenance and also lower acquisition costs.

To ensure the required safety level for people when producing the weldseams by the amplified signal light, whether these are continuously orspot-welded, the position of the collimator is so controlled that thelaser beam can only exit from the housing through the exit slit. Inaddition, the laser output of the fiber laser is activated only when theexit slit sits upon the work piece. By combining these measures, theweld seam can be produced along the predetermined line of motion andwithout the laser welding tool putting people in the working environmentof the robot at risk of being harmed, even if the tool is mounted on arobot arm. Obstruction of the welding process by the housing edges ismainly prevented if the housing is tapered towards the exit slit.

Conventional resistance welding systems in the for of spot-welding tongsare typically moved by a welding robot in the automotive industry. Tomaintain these operating processes without change, the laser weldingtool is preferably configured as welding tongs with a holder, a C-shapedlower tool attached to the holder, and a traversing unit that moves thehousing relative to the holder, wherein a pressure piece is arranged inalignment with the free end of the lower tool and the pressure piece andthe exit slit that is in alignment with it can be moved together andapart using the traversing unit. The pressure piece forms the thrustbearing during the force-driven approximation of the housing and lowertool. Optimized load transmission when the welding tongs collapses isachieved by matching the pressure piece to the shape of the exit slit.

For modular design and cost reduction of the laser weld tool accordingto the disclosure, the traversing unit comprises an actuator mounted tothe holder, a receptacle for the housing, and a linear guide for thehousing receptacle. Preferably, the housing receptacle is configuredsuch that various housings can be installed on the housing receptacle ina short time. Adjustment to different welding conditions is thereforeeasily possible.

The housing includes at least a collimator, the linear drive associatedwith the collimator, and the especially funnel-shaped enclosure of thelaser beam. For smooth movement of the collimator and the laser beam andexact control of starting travel and maintaining specified collimatorpositions, the linear drive for the collimator comprises a single-axislinear carriage that is connected to the actuator. The actuator istypically configured as a servomotor. Electric and hydraulic motors aresuitable.

A particular switching element provided on the housing switches thepower supply of the fiber laser on when the exit slit is sitting on oneof the work pieces and thus activates the fiber laser in that position.The switching element can be a contact directly adjacent to the exitslit, a sensor element, or a pushbutton. Alternatively, a movable partof the housing that includes the exit slit can also function as aswitching element.

To quickly bring the opened laser welding tongs in position on the workpieces to be welded using a robot, the holder in a preferred embodimentof the disclosure is arranged on a compensating module that allows minorcompensatory movements of the holder relative to a robot arm for movingthe laser welding tool. When the tool travels to the welding position,the pressure piece of the lower tool is at a defined distance from oneof the work pieces, for example, 2 to 3 mm. The exit slit of the lasermodule will be, for example, 150 mm away from the work piece to bewelded when the welding tongs is open. Only after moving into thewelding position, first the lower tool and then the housing (upper tool)are brought into contact on the work pieces using the compensatingmodule. Slightly varying positions of the work pieces to be welded withrespect to a defined welding position of the welding tongs do thereforenot pose as problem for traveling into welding position.

For constant floating alignment of the lower tool of the laser weldingtool to the work piece without changing the position of the work piece,one embodiment of the disclosure contemplates that the compensatingmodule comprises a compensating base to be mounted to a robot arm, thata linear guide for the holder arranged for the same direction of travelas the linear guide of the housing receptacle is arranged on saidcompensating base, and that the compensating module comprises ashort-stroke linear motor that acts on the holder and has aspring-driven unit incorporated into its linear-acting flow of force.

In one embodiment of the disclosure, a wobble generator that can bemoved along the predetermined line of motion is arranged between thecollimator and a supporting member of the linear drive to produce awider weld seam. A wobble generator (also called a wobblers) is anelectronic device for generating wobbles wherein the frequency generatedvaries cyclically between end values that can be set. The wobbles may begenerated, for example, by electromagnetic, electromotive, orpiezoelectric means. Controlled triggering of the wobble generatorallows the collimator to perform an oscillating motion at variableamplitudes and frequencies perpendicular to the predetermined line ofmotion. The supporting member of the wobble generator may for example bea traveling base plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be explained in more detail below with reference tothe figures. Wherein:

FIG. 1 a is a perspective view of a laser welding tongs according to thedisclosure in its open state.

FIG. 1 b shows a laser welding tongs as in FIG. 1 a in its closed state

FIG. 2 a is a lateral view and a top view of an opened laser weldingtongs according to FIG. 1

FIG. 2 b is a lateral view and a top view of an closed laser weldingtongs according to FIG. 1, and

FIG. 3 is a diagrammatic view of the operating principle of a laserwelding tongs according to FIGS. 1 and 2.

SPECIFIC DESCRIPTION

FIG. 1 a shows a laser welding tongs 1 with a holder 2 configured as aplate, a generally C-shaped lower tool 3 arranged on the holder 2, witha pressure piece 5 mounted to the free end 4 of the lower tool 3. Atraversing unit 6 is arranged on the holder 2 and moves the laser modulereferenced as a whole as 7 into the direction of the pressure piece 5 oraway from it.

FIG. 3 illustrates the design and operating principle of the lasermodule 7, which is configured to as a fiber laser. The pump radiation iscoupled into a fiber 8 by means of focusing optics, which is not shown,but well known to one of ordinary skills in the art. The laser moduleincludes a collimator 9 receiving the fiber 8 for focusing the laserradiation 11. The linear drive 12 for the collimator 9 comprises alinear carriage 14 that can be moved along the only axis 13 and, in theembodiment shown, is connected to an actuator 16 such as a servomotorvia a belt and chain drive 15. The linear drive 12 with the collimator 9including a portion of the fiber 8 are arranged in a housing 18comprising an exit slit 17 for the laser radiation 11 that tapers like afunnel towards the exit slit 17 in a section 19 of a housing extension21. The laser radiation 11 is fully enclosed in the housing 18,particularly its extension 21 with the funnel-shaped section. Thisensures that the laser radiation 11 can only exit from the exit slit 17.

The laser energy for activating the fiber laser is activated by apressure piece 22 located at the front end of the tapering section 19 inthe following manner. When tapering section 19 abuts one of the workpieces, a power circuit of the laser system is closed which causes thegeneration of laser radiation. In addition, the power circuit activationcan be made dependent on the buildup of a sufficient pressure forcebetween the pressure pieces 5 and 22 of respective tool 3 and housing 18which are positioned in alignment with one another. The force ismeasured by force sensors in the flow of force of the tongs, e.g. on orin the extension of the lower tool 3.

Referring to FIG. 1 b in addition to FIG. 3, the traversing unit 6comprises an actuator 23 in form of a servomotor attached to the holder2, a housing receptacle 24, and a linear guide 25 configured as a guiderail for the housing receptacle 24. The actuator 23 is attached by meansof a bracket 26 (FIG. 3) to the holder 2.

As can be seen from FIG. 1 b, the housing receptacle 24 is configured asa rectangular profile, and a fixing bracket 27 onto which a drive shaft28 of the servomotor 23 acts is attached to its front end that faces thelower tool. Guide elements 29 are provided on the bottom side of thehousing receptacle 24 and engage in corresponding grooves 31 of theguide rail 25. The laser module 7 can be moved along a freelyprogrammable path 32 up to the work piece using the actuator 23 of thetraversing unit 6.

Returning to FIG. 3, a compensating module 33 that can be moved relativeto the holder 2 is arranged on the bottom side of the plate-shapedholder 2 and allows slight compensatory movements of the holder 2relative to a robot arm 34. The robot arm (34) is connected via aquick-connector flange 35 to the compensating module 33 of the weldingtongs 1.

The compensating module 33 is configured with a compensating base 36having a linear guide 37 for the holder 2 and attached to thequick-connector flange 35. The linear guide 37 allows the same directionof movement as the linear guide 25 of the housing receptacle 24.

A fastening flange 38 is provided on the end of the lower tool 3opposite the holder 2. A short-stroke cylinder 39 that acts on theholder 2 and a spring-pressure unit 41 are inserted between thefastening flange 38 and the compensating base 36. The spring pressureunit 41 is integrated into the piston rod of the short-stroke cylinder39 such that the linear-acting flow of force is not impaired. For thispurpose, a compression spring encompassed by a sleeve can be integratedas a spring-pressure unit into the piston rod.

FIGS. 2 a and 2 b illustrate the operating mode of the laser weldingtongs 1 and the compensating module 33. FIG. 2 a shows how the openedlaser welding tongs 1 held by the robot arm 34 of a robot is broughtinto position, at work pieces 10 to be joined. The lower tool 3 that isattached to the holder 2 will be at a defined distance 42 from the workpiece of, for example, 2 to 3 mm when the compensating module 33 is notactivated. This distance 42 is selected to allow the robot to move fastinto welding position at any time, taking into account any work pieceposition variations. The distance 43 of the laser module 7 from the workpiece can be freely programmed within the overall stroke of thetraversing unit 6 and may, for example, be 150 mm.

The closing function of the laser welding tongs 1 is started when thelaser welding tongs 1 are positioned at the work pieces 10. Initially,the compensating module 33 is activated by activating the short-strokecylinder 39 (FIG. 3) that presses the lower tool 3 with the pressurepiece 5 with a defined force against the work piece 10. This force isdimensioned such that the lower tool 3 aligns in a floating manner withthe work piece but without changing the position of the work piece. Ifthe position of work piece 10 varies, the spring-force unit 41 willensure floating alignment by a defined force. The traversing unit 6 isstarted at the same time as the compensating module 33 is activated. Dueto the greater distance 43 from the work piece 10, the laser module withthe exit slit 17 comes to rest against the work piece 10 somewhat laterwhen the traveling speed is the same. While the tongs closes, thepressure piece 5 of the lower tool 3 and the pressure piece 22 (FIG. 2b) of the laser module 7 come to rest in alignment on the work pieces10. The welding process can only start after the pressure piece 22 ofthe laser module 7 sits upon the work piece.

As the collimator 9 moves along the linear carriage 14, the laser beam11 that is focused towards the exit slit 17 moves in parallel to theline of motion 13 of the collimator. A continuous or spot-welded seamcan he laid along length of the exit slit into the surface section ofthe work piece 10 around the exit slit 17.

For laying a wider weld seam, a wobble generator 13 a (FIG. 3) may bearranged between the collimator 9 and the supporting member of thelinear carriage 14 that can be moved along the predetermined line ofmotion 13. The wobble generator 13 a generates wobbles of the collimator9 that is pivoted about an axis of rotation. The axis of rotationextends in parallel to the traversing path 13 of the collimator 9. Thewobbles cause the laser beam 11 on the exit slit 17 to oscillate at anamplitude matching their width, producing a weld seam that is up to 2 mmwider.

Although shown and disclosed is what is believed to be the mostpractical and proffered embodiments, it is apparent that departures fromthe disclosed configuration will suggest themselves to those skilled inthe art. Accordingly, the present invention is not restricted to theparticular constructions described and illustrated, but should beconstrued to cohere with all modifications that may fall within thescope of the appended claims.

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 10. (canceled) 11.A laser welding method for joining work pieces in an eye-safeenvironment, the method comprising: providing a laser module including acollimator, a delivery fiber, a linear drive, and a housing, thecollimator configured to receive the delivery fiber for delivering laserradiation to a work piece, the housing configured to provide an exitslit for the laser radiation such that the laser radiation may only exitfrom said exit slit and the linear drive is configured to move thehousing and thereby the exit slit in proximity to a desired weldinglocation; providing a pressure piece located proximate to the exit slitand configured to provide a closed circuit control of emission of laserradiation when actuated; abutting the pressure piece to a work piecesufficient to actuate the pressure piece and close the circuit; andproviding laser radiation to the work pieces sufficient to create a weldwhile maintaining an eye-safe environment.
 12. The method of claim 1,wherein the work pieces to be welded are metal.
 13. The method of claim1, wherein a weld seam is produced along a predetermined line of motionwithin the exit slit.
 14. The method of claim 1, wherein a spot weld isproduced.
 15. The method of claim 1 further comprising the provision ofa wobble generator to allow for oscillation of the laser radiationduring the welding of the work pieces.
 16. The method of claim 1 furthercomprising the provision of a robot to move the laser module to apre-determined welding location.